There are two major categories of high accuracy frequency standards: (a) atomic frequency standards, and (b) stand-alone crystal oscillators. In general, the higher the inherent accuracy of any frequency source, the higher the cost, size and weight. The currently used precision frequency sources in descending order of accuracy are: (a) cesium frequency standards, (b) rubidium frequency standards, (c) oven controlled crystal oscillators and (d) temperature compensated crystal oscillators. The cesium and rubidium standards are termed "atomic frequency standards". Oven controlled crystal oscillators and temperature compensated crystal oscillators will be termed "stand-alone crystal oscillators" in this application.
Most atomic frequency standards incorporate a slave crystal oscillator. The resulting combination is designed to take advantage of the long term stability of the atomic resonator and the short term stability of the slave crystal oscillator. The output frequency of the combination is taken from the slave crystal oscillator (often via a frequency synthesizer).
The frequencies of frequency standards change due to the influences of temperature, shock, vibration, or ionizing radiation. Furthermore, the frequencies of rubidium frequency standards, oven controlled crystal oscillators and temperature compensated crystal oscillators also change with time, i.e. they age. In all currently-available frequency standards, some means is provided for adjusting the frequency of the standard in order to eliminate undesirable frequency offsets. For example, in a rubidium standard, a magnetic field, termed the "C-field" may be adjusted in order to cause a slight change in the rubidium frequency standard output frequency. In stand-alone crystal oscillators, a varactor or other control circuitry is usually incorporated to provide output frequency adjustment.
An external reference frequency standard is needed to calibrate secondary frequency standards such as a rubidium standard or stand-alone crystal oscillators. Common sources of external reference frequency standards are: (a) a portable cesium standard, (b) a Loran-C receiver, and (c) a Global Positioning System (GPS) receiver. During calibration, the frequency of the frequency standard is adjusted to that of one of the above-mentioned external frequency references, i.e. the frequency source is syntonized to the external frequency reference. U.S. Pat. No. 4,476,445, entitled "Method and Apparatus for Rapid and Accurate Frequency Syntonization of an Atomic Clock" issued to Riley discloses a method for syntonizing the frequencies of a laboratory frequency standard with an external reference standard.
Many of the prior art methods for calibrating atomic frequency standards (i.e. syntonizing the frequencies of a frequency standard with an external reference standard) involve adjustment of the magnetic field termed the "C-field." For example, the aforementioned U.S. patent provides for an automatic C-field adjustment. The requirement to periodically adjust the C-field of a laboratory atomic frequency standard means added complexity and cost and provides a source of potential instability for the laboratory atomic frequency standard. Moreover, in rubidium frequency standards, the available C-field adjustment range limits the useful life of the unit. For example, in one of the most popular rubidium frequency standards currently on the market, the manufacturer provides a C-field adjustment range equivalent to +1.5.times.10.sup.-9. The aging rate of the standard is specified as 2.times.10.sup.-10 per year. Consequently, at the specified aging rate, the limited C-field adjustment range limits the useful life of this rubidium frequency standard to 1.5.times.10.sup.-9 /2.times.10.sup.-10 =7.5 years.
Consequently, those concerned with the maintenance of laboratory atomic frequency standards have continuously sought improved apparatus and techniques for syntonizing the frequencies of laboratory standards with available external references.